The present invention relates to a lifting mechanism and to a method for triggering a lifting mechanism.
Modern tractors, besides their conventional task as a pulling machine, also take on the tasks of supplying energy to and controlling attachments that are suspended from or semimounted on a lifting mechanism. This last function is accomplished on the one hand by a power takeoff shaft of the tractor or by the work hydraulics. In tractors of the medium and upper power categories, electrically triggerable multi-position valves for controlling the work functions of the attachments are being used more and more. The control of these hydraulic functions is typically effected via a compact control block, in which the control is combined in a single unit with all the essential multi-position valves and regulating valves. One such control block, intended for so-called electrohydraulic lifting mechanism regulation (EHR), is described for instance in the data sheet RD 66 125/09.04 (EHR-Ventile [EHR Valves]) published by Bosch Rexroth AG.
In German Patent Application DE 10 2005 005 314 A1, a double-action lifting mechanism is described, with which an attachment can be triggered actively in the “lifting” (carrying) direction or in the “pressing” (lowering) direction. The working range for “carrying” occurs for instance in plowing or cultivating. The working range for “pressing” is set for instance in active plow retraction or in a packer. In a load-free intermediate position, in which the lifting mechanism is not subjected to a force, the work unit rests on the ground by its own weight. This intermediate position is set in a floating position of the lifting mechanism valve.
The pressure that is operative in the “pressing” direction is limited, in the known version, via a proportionally variable pressure limiting valve on the basis of the setting by the driver and the operating state of the EHR.
In some applications, such as mowing, it is desirable to adapt the force with which of the work unit rests on the ground. For instance, a mowing mechanism should rest on the ground with only a very slight load, since then the turf is not damaged, and fuel economy is made possible as well.
For regulating this kind of relief pressure, it is proposed in German Patent Disclosures DE 36 28 427 A1 and DE 195 41 190 A1 that the pressure operative in the “carrying” direction be limited via a proportionally variable pressure limiting valve, so that the relief pressure cannot exceed the preset maximum pressure. It is a disadvantage of this version is that in this relief pressure regulator, a large volumetric flow of pressure fluid flows away to the tank, via the opened metering aperture of the lifting mechanism valve and via the pressure limiting valve, and thus on the one hand, pressure fluid becomes heated, and on the other, hydraulic energy is wasted. In the first of these two references, the problem is that the snow plow cannot move lower and hence is unable to follow the contour of the ground.
Versions are also known in which relief pressure regulation is done by activating two hydraulic reservoirs as a function of the signal of a pressure sensor. In alternative versions, it is proposed that the work unit, such as a mowing mechanism, be relieved via a spring assembly. Such versions are quite complex and expensive and require considerable space. A further disadvantage of spring relief is that during the work process, no variation of the forces operative in the “carrying” direction is possible, and thus these systems lack the flexibility desired for high-quality tractors.